Steel wire rod having coating film that has excellent corrosion resistance and workability, and method for producing same

ABSTRACT

The present invention provides a steel wire rod having a lubricating coating film, which can achieve both workabilities such as wire drawability, spike property and ball ironing property, and corrosion resistance such as long-term rust prevention property; and a method for producing the same. Disclosed are a steel wire rod having a coating film containing no phosphorus, wherein the coating film includes a lower layer coating film composed of oxide and/or hydroxide of zirconium and having a film thickness of 1.0 to 200 nm, and an upper layer coating film containing silicon and tungsten, in order from a steel wire rod side, a mass ratio of tungsten/silicon being in a range of 1.3 to 18; and a method for producing the above steel wire rod, which includes bringing an aqueous chemical conversion treatment solution, which has a pH in a range of 2.5 to 5.0 and contains a water-soluble zirconium compound dissolved therein, into contact with a surface of a steel wire rod to form a lower layer coating film.

TECHNICAL FIELD

The present invention relates to a steel wire rod having a coating film containing no phosphorus on a surface, and a method for producing the same.

BACKGROUND ART

In plastic working of a steel wire and a steel wire rod, since friction generated when surfaces of metals (particularly a die and a workpiece) are violently rubbed against each other may cause an increase in working energy, heat generation, seizure phenomenon, and the like, there have been used various lubricants which aim to reduce a friction force. Oils, soaps, and the like have been used as the lubricant for a long time, and the friction force has been reduced by supplying them to a friction surface to form a fluid lubricating coating film. In plastic working in which sliding occurs under high surface pressure involving significant heat generation due to an increase in surface area, a seizure phenomenon is likely to generate due to shortage of lubrication, lubricating coating film disruption, and the like. Therefore, there has been popularized technology in which a surface of a metal material is coated in advance with a solid coating film, for example, an inorganic coating film such as a borate coating film or a phosphate crystal coating film, which has sufficient coating film strength and exists at an interface between a die and a workpiece and is therefore less likely to cause lubricating coating film disruption even under high surface pressure, thus making it possible to avoid direct contact between metals. In particular, a composite coating film composed of a zinc phosphate coating film and a soap layer (hereinafter sometimes referred to as a chemical conversion coating film) has widely been employed because of having high workability and corrosion resistance.

In recent years, there have been surging a wide range of requirements for a solid coating film, for example, further reduction working energy and increase in working degree, coping with a hard-to-work material, environmental protection of a coating film process (for example, a phosphatizing treatment has an environmental conservation problem because of generation of numerous industrial wastes such as sludge), taking measures to phosphorizing of a bolt (if phosphorus in a coating film component remains during a heat treatment after heading of a high strength bolt, phosphorus enters into a steel, thus causing brittle fracture), and the like. While global environment conservation is taken into consideration to these requirements, a solid coating film having high lubricity has been developing. This technology enables formation of a coating film having high lubricity by a simple step of only applying an aqueous plastic working lubricant to a surface of a workpiece, followed by drying.

Patent Document 1 discloses an aqueous lubricating coating agent for plastic working of a metal material, which is a composition comprising an aqueous inorganic salt (A) and a wax (B) dissolved or dispersed in water, wherein a solid component weight ratio (B)/(A) is in a range of 0.3 to 1.5; and a coating film forming method thereof.

Patent Document 2 discloses an aqueous lubricating coating agent for plastic working of a metal material, comprising an alkali metal borate (A), wherein the alkali metal borate (A) contains lithium borate, a molar ratio of lithium to the entire alkali metal in the alkali metal borate (A) is in a range of 0.1 to 1.0, and also a molar ratio (B/M) of boric acid B to an alkali metal M of the alkali metal borate (A) is in a range of 1.5 to 4.0; and a coating film forming method thereof. This technology suppresses crystallization of a coating film caused by moisture absorption of the coating film, thus enabling formation of a coating film having not only workability but also high corrosion resistance.

Patent Document 3 discloses a non-phosphorus based water-soluble lubricant for plastic working, comprising an inorganic solid lubricant as a component A, a wax as a component B, and a water-soluble inorganic metal salt as a component C, wherein a solid component mass ratio of the component A to the component B (component A/component B) is in a range of 0.1 to 5, and a solid component mass ratio of the component C to the total amount of the component A, the component B and the component C (component C/(component A+component B+component C)) is in a range of 1 to 30%. It is considered that this technology is directed to a lubricant containing no phosphorus and enables realization of corrosion resistance equal to that of a chemical conversion coating film.

Patent Document 4 discloses an aqueous lubricating coating agent comprising an aqueous inorganic salt (A), a lubricant (B) which is at least one selected from molybdenum disulfide and graphite, and a wax (C), these components being dissolved or dispersed in water, wherein (B)/(A) is in a range of 1.0 to 5.0 in terms of a solid component weight ratio, and (C)/(A) is in a range of 0.1 to 1.0 in terms of a solid component weight ratio; and a coating film forming method thereof. This technology enables realization of high workability having the same level as that of a chemical conversion coating film by mixing a conventional aqueous lubricating coating agent with molybdenum disulfide or graphite.

Patent Document 5 discloses a coating film forming agent comprising a silicate (A), a polycarboxylate (B), a hydrophilic polymer and/or a hydrophilic organic lamellar structure (C), and a molybdate and/or a tungstate (D), a mass ratio of each component being a predetermined ratio.

As mentioned in Patent Documents 1 to 5, the aqueous inorganic salt is an essential component in the solid coating film of the aqueous lubricating coating agent. The reason is that the lubricating coating film composed of the aqueous inorganic salt has sufficient coating film strength and, as mentioned above, the lubricating coating film exists at an interface between a die and a workpiece and is therefore less likely to cause lubricating coating film disruption even under high surface pressure, thus making it possible to avoid direct contact between metals. Therefore, in the aqueous lubricating coating agent, it is possible to maintain a satisfactory lubricated state during plastic working by using a solid coating film composed of an aqueous inorganic salt or a water-soluble resin in combination with an appropriate lubricant capable of reducing a friction coefficient.

A description will be made of coating film formation mechanism of the aqueous lubricating coating composed of a water-soluble component. An aqueous inorganic salt of a water-soluble component is in a state of being dissolved in water in a lubricating treatment solution and, when a lubricant is applied on a surface of a metal material and then dried, water as a solvent is vaporized to form a lubricating coating film. In that case, the aqueous inorganic salt is precipitated as a solid substance on the surface of the metal material to form a solid coating film. The solid coating film thus formed has a coating film strength capable of enduring plastic working, and exhibits satisfactory lubricity during plastic working by mixing with an appropriate lubricant capable of reducing a friction coefficient.

PRIOR ART DOCUMENT Patent Document

Patent Document 1: WO 02/012420 A

Patent Document 2: JP 2011-246684 A

Patent Document 3: JP 2013-209625 A

Patent Document 4: WO 02/012419 A

Patent Document 5: JP 2002-363593 A

DISCLOSURE OF THE INVENTION Problems to be Solved by the Invention

However, in the lubricating coating films of Patent Documents 1 to 5, rust prevention property over a long term of four or more months is drastically inferior as compared with the above-mentioned chemical conversion coating films, thus failing to enhance to a practical level. This is because a main component of the coating film is a water-soluble component and therefore easily absorbs or transmits moisture in the atmosphere, leading to easy contact between a steel material and moisture. In Patent Document 2, corrosion resistance is improved by suppressing crystallization of the coating film due to moisture absorption, however, moisture absorption itself is not suppressed, thus failing to obtain sufficient corrosion resistance. It was mentioned that the aqueous lubricating coating film mentioned in Patent Document 3 exhibited corrosion resistance, which is equal to or better than that of the chemical conversion coating film, in a corrosion resistance test in a laboratory in which rusting is accelerated using a thermo-hygrostat. Commonly, the lubricating coating film is actually used in the environment where dusts and powders, and mists of a picking agent are adhesible. In such severe environment, corrosion resistance is actually inferior as compared with the chemical conversion coating film. As mentioned above, there has never been an aqueous lubricating coating film containing no phosphorus, having rust prevention property which is equal to or better than that of the chemical conversion coating film.

The aqueous lubricating coating film is inferior to the chemical conversion coating film in nonuniformity of the coating film. For example, if there are some positions where wire rods are laid one upon another or binding positions, when a wire rod coil is treated in a batchwise manner, a film treatment solution does not spread to the positions, leading to formation of a thin coating film. Particularly, since the thickness of the coating film exerts a significant influence on corrosion resistance, for example, there is recognized a phenomenon in which rusting starts from the binding position of the coil. Degradation of corrosion resistance due to nonuniformity of this film was a great problem for a conventional aqueous lubricating coating film.

Examples of the aqueous inorganic salt capable of obtaining comparatively high corrosion resistance include an alkali metal salt of a silicate (hereinafter sometimes referred to as a silicate) and an alkali metal salt and/or an ammonium salt of a tungstate (hereinafter sometimes referred to as a tungstate). These aqueous inorganic salts are also mentioned in Patent Document 1, Patent Document 4 and Patent Document 5. However, they are far inferior in practical corrosion resistance as compared with the chemical conversion coating film.

The water-soluble silicate has a property that is less likely to transmit moisture among the water-soluble aqueous inorganic salts and also has very high adhesion to a material. Because of this property, it is a material that can exhibit comparatively high corrosion resistance, but not as much as the chemical conversion coating film. This is because the water-soluble silicate is crosslinked to from a network structure in a coating film formation process in which water as a solvent of a lubricant is vaporized. However, because of this network structure, the coating film of the water-soluble silicate is too brittle as a lubricating coating film. Therefore, when a base material is worked, it is sometimes impossible to sufficiently conform because of cracks of the coating film. Too high adhesion due to the network structure may cause insufficient film removal, thus resulting in various defects in the subsequent step. For example, when plating is performed in the subsequent step, inclusion of a coating film component may cause not only contamination of a plating solution, but also poor plating in the portion where the coating film component remains.

The water-soluble tungstate is less likely to absorb moisture from external air when a coating film is formed. This is because granular crystals are formed when the water-soluble tungstate forms a coating film. Further, the water-soluble tungstate has a property that forms a passive coating film having a self-repair function on a surface of a metal material, and use of the water-soluble tungstate as the coating film component enables expectation of formation of a coating film having high corrosion resistance. Because of its high water solubility, it is possible to easily perform film removal with an aqueous solution. However, the water-soluble tungstate is crystalline and is therefore inferior in adhesion to a material and cannot form a uniform coating film, thus failing to obtain corrosion resistance and workability as expected. For example, it is possible to enhance adhesion and uniformity of a coating film by adding a synthetic resin component in a lubricant, but the corrosion resistance is drastically inferior as compared with a chemical conversion coating film.

The aqueous lubricating coating agents containing an aqueous water-soluble inorganic salt mentioned in Patent Documents 1 to 3 are commonly inferior in workability as compared with a chemical conversion coating film. This tendency is particularly notable in severe working wherein a surface area expansion ratio becomes at least several tens of times (hereinafter sometimes referred to as severe working), thus causing insufficient deformation of a material, decrease in die life, occurrence of seizure, and the like.

In the aqueous lubricating coating agent mentioned in Patent Document 4, it is possible to obtain workability, which is equal to or better than that of a chemical conversion coating film, even during severe working by inclusion of molybdenum disulfide and graphite. However, inclusion of these components colors a lubricating liquid black, leading to extreme contamination of an apparatus and peripheral parts, and operators. Molybdenum disulfide and graphite are likely to be sedimented and may be sometimes coagulated with the lapse of time on the bottom of a treatment tank, thus making it difficult to disperse again. Therefore, it is difficult to perform a stable operation. Further, these two components may cause significant degradation of corrosion resistance, so that the resulting coating film is inferior in corrosion resistance as compared with the lubricating coating films of Patent Documents 1 to 3, to say nothing of the chemical conversion coating film.

In Patent Document 5, a coating film treatment agent containing a silicate (A) as a main component and containing excessively large amount of an anti-corrosive agent (D) is inferior in lubricity since seizure occurs under high extrusion load. Therefore, it becomes difficult to perform stable operation, thus failing to obtain sufficient long-term rust prevention property.

As mentioned above, it was impossible for the aqueous lubricating coating film to form a coating film that can achieve both high corrosion resistance over a long term of about four or more months comparable to that of the chemical conversion coating film even in a service environment, and workability during severe working. Particularly, with respect to corrosion resistance, rusting from the position of the thin coating film becomes a problem since the coating film is likely to become nonuniform. When a silicate is contained in the aqueous lubricating coating agent, insufficient film removal becomes a problem.

Thus, it is an object of the present invention to provide a steel wire rod having a lubricating coating film that can achieve both workabilities such as wire drawability, spike property and hall ironing property, and corrosion resistance such as long-term rust prevention property; and a method for producing the same.

Means for Solving the Problems

The inventors of the present invention have intensively studied so as to solve the above problems and found that it is possible to achieve both workability and corrosion resistance when the below-mentioned upper layer coating film and lower layer coating film are formed on/over a steel wire rod as a lubricating coating film. Specifically, they have found that it is possible to obtain high corrosion resistance and workability that have never been achieved by each component mentioned below alone, by forming, as a lower layer coating film, a film made of oxide and/or hydroxide of zirconium, and adjustment of a ratio of a silicate to a tungstate to a certain specific ratio, namely, adjustment of a mass ratio of tungsten/silicon to a predetermined ratio to form a composite upper layer coating film, and thus the present invention has been completed.

To solve the above problems, the present invention was structured as mentioned below.

A gist of the steel wire rod of the present invention lies in a steel wire rod having a coating film containing no phosphorus, wherein the coating film includes a lower layer coating film composed of oxide and/or hydroxide of zirconium and having a film thickness of 1.0 to 200 nm, and an upper layer coating film containing silicon and tungsten, in order from a steel wire rod side, a mass ratio of tungsten/silicon being in a range of 1.3 to 18.

It is preferred that the silicon is derived from water-soluble silicate, and the tungsten is derived from water-soluble tungstate.

It is preferred that the silicon is derived from at least one selected from lithium silicate, sodium silicate and potassium silicate, and the tungsten is derived from at least one selected from lithium tungstate, sodium tungstate, potassium tungstate and ammonium tungstate.

It is preferred that a resin is contained in the upper layer coating film, and a mass ratio of resin/(silicon+tungsten) is in a range of 0.01 to 3.2.

The resin is preferably at least one selected from a vinyl resin, an acrylic resin, an epoxy resin, a urethane resin, a phenol resin, a cellulose derivative, a polymaleic acid and a polyester resin.

It is preferred that a lubricant is contained in the upper layer coating film, and a mass ratio of lubricant/(silicon+tungsten) is in a range of 0.01 to 3.2.

The lubricant is preferably at least one selected from wax, polytetrafluoroethylene, fatty acid soap, fatty acid metal soap, fatty acid amide, molybdenum disulfide, tungsten disulfide, graphite and melamine cyanurate.

The mass of the coating film per unit area of the upper layer coating film is preferably in a range of 1.0 to 20 g/m².

A gist of the method for producing a steel wire rod of the present invention lies in a method which includes bringing an aqueous chemical conversion treatment solution, which has a pH in a range of 2.5 to 5.0 and contains a water-soluble zirconium compound dissolved therein, into contact with a surface of a steel wire rod to form a lower layer coating film.

Effects of the Invention

Since a lubricating coating film including an upper layer coating film and a lower layer coating film was structured as mentioned above in the steel wire rod of the present invention, it is possible to obtain a steel wire rod that is excellent in workabilities such as wire drawability, spike property and ball ironing property, as well as corrosion resistance such as long-term rust prevention property. The steel wire rod of the present invention is excellent as compared with a conventional aqueous lubricating coating film in that all of these performances are equal to or better than those of a steel wire rod including a chemical conversion coating film. It is a feature that did not exist in a conventional aqueous lubricating coating film that it is possible to obtain high corrosion resistance even when an aqueous lubricating coating film became thin due to external factors such as overlapping and binding between materials.

MODE FOR CARRYING OUT THE INVENTION

The present invention is directed to a steel wire rod having a coating film containing no phosphorus, wherein the coating film includes a lower layer coating film composed of oxide and/or hydroxide of zirconium and having a film thickness of 1.0 to 200 nm, and an upper layer coating film containing silicon and tungsten, in order from a steel wire rod side, a mass ratio of tungsten/silicon being in a range of 1.3 to 18.

In the present invention, a steel used in the steel wire rod also includes a carbon steel, an alloy steel, a special steel, and the like. Examples of such steel include a mild steel having a carbon content of 0.2% by mass or less (not including 0% by mass), a carbon steel having a carbon content of exceeding 0.2% by mass and 1.5% by mass or less, and an alloy or special steel containing at least one selected from silicon, manganese, phosphorus, sulfur, nickel, chromium, copper, aluminum, molybdenum, vanadium, cobalt, titanium, zircon, and the like according to the application of the carbon steel. In the present invention, the steel wire rod generally refers to those obtained by forming a steel into a wire rod through hot working. The steel wire is included in the steel wire rod of the present invention. The steel wire refers to those obtained by further subjecting a steel wire rod to a working treatment, such as those obtained by drawing a steel wire rod into a wire having a specified size (wire diameter, circularity, etc.) and those obtained by subjecting a steel wire rod or a steel wire drawn into a wire to a plating treatment.

The steel wire rod of the present invention includes a lubricating coating film composed of at least two layers, namely, a lower layer coating film and an upper layer coating film in order from a surface of a steel wire rod. Each of the upper layer coating film and the lower layer coating film may be a single layer, or a layer composed of two or more layers. If necessary, a layer may be further formed on the upper layer coating film, between the upper layer coating film and the lower layer coating film, or between the steel wire rod and the lower layer coating film.

Both of the coating films contain no phosphorus, and a composition used for formation of the film does not contain a component containing phosphorus. However, in the present invention, it is not excluded that a component containing phosphorus is inevitably included in a coating film of a surface of the steel wire rod in the operation process. Namely, even if phosphorus as inevitable impurities may cause contamination in the actual operation, there is little possibility that phosphorus causes brittle fracture of a steel wire rod when the content of phosphorus is about 1% by mass or less, and thus it is possible to consider that phosphorizing does not occur.

A description will be made in order from each component, each composition, and the like of a lubricating coating film in a steel wire rod according to the present invention.

There is a need that the upper layer coating film is a film that contains silicon and tungsten, a mass ratio of tungsten/silicon being in a range of 1.3 to 18. Inclusion of silicon and tungsten in this range enables formation of a film having high corrosion resistance, workability and sufficient adhesion that have never been realized by the below-mentioned silicate or tungstate alone, or the other aqueous inorganic salt.

For example, when the below-mentioned water-soluble silicate and water-soluble tungstate are composited to form a coating film, the tungstate is incorporated into a network structure formed of the silicate. As mentioned above, drawbacks of the tungstate depend heavily on formation of a crystalline coating film and it becomes possible for the tungstate to exist uniformly and finely by incorporating into the network structure of the silicate. Whereby, it is possible to achieve both property of being less likely to transmit moisture of the silicate and a passive film having a self-repair function of the tungstate, leading to a remarkable improvement in corrosion resistance.

Examples of the influence of the tungstate on the include an improvement in film removability. As mentioned above, the silicate is inferior in workability and film removability since a firm continuous film is formed by polymerization of the silicate. The composited tungstate exists in the network structure of the silicate, whereby, formation of a firm network structure is appropriately suppressed, thus enabling an improvement in workability and film removability.

A mass ratio tungsten/silicon is 1.3 or more, preferably 1.8 or more, and still more preferably 2.0 or more. The mass ratio is 18 or less, preferably 10 or less, and more preferably 5.4 or less.

If the mass ratio of tungsten/silicon is less than 1.3, the obtained film can achieve neither sufficient corrosion resistance nor workability, and is also inferior in film removability. This is because the amount of the tungstate relatively decreases, thus failing to sufficiently form a passive film, while the amount of the silicate relatively increases to form a firm network structure. If the mass ratio of tungsten/silicon is more than 18, the obtained film can achieve neither sufficient corrosion resistance nor workability. This is because the amount of the silicate relatively decreases, thus making it easier to transmit moisture, while crystals of tungsten are precipitated, thus degrading adhesion and uniformity of the film. In the present invention, the mass ratio of tungsten/silicon is preferably based on a ratio of tungsten element derived from the water-soluble tungstate to a silicon element derived from the water-soluble silicate in the film, and the ratio can be calculated as mentioned later.

The lower layer coating film is made of oxide and/or hydroxide of zirconium, and preferably oxide of zirconium. In the present invention, the lower layer coating film is sometimes a zirconium coating film.

The film thickness of the lower layer coating film is 1.0 nm or more, preferably 5 nm or more, and more preferably 20 nm or more. The film thickness is 200 nm or less, preferably 150 nm or less, and more preferably 130 nm or less. The film thickness of less than 1.0 nm leads to too thin zirconium coating film, thus failing to exhibit sufficient corrosion resistance. Whereas, the film thickness of more than 200 nm exerts no influence on corrosion resistance, but poor adhesion to the coating films is exhibited, thus degrading workability.

Formation of a zirconium coating film in such a manner enables an improvement in corrosion resistance. This is because further formation of a zirconium coating film as a lower layer coating film leads to a change of a network structure of the silicate, whereby, not only binding is enhanced, but also corrosion resistance of the binding enhancement part is further improved because of having moisture blocking capability. The zirconium coating film is a very thin film as compared with the upper layer coating film and is therefore likely to form a uniform film even at a point where a conventional aqueous lubricating coating film is likely to become a thin film, and this leads to an improvement in corrosion resistance even at such a point. As mentioned above, since the network structure of the upper layer coating film and the binding enhancement portion of a zirconium coating film are important for an improvement in corrosion resistance, an influence of a film thickness of the entire upper layer coating film is less likely to be exerted. Therefore, corrosion resistance can be sufficiently exhibited even in the point where the above-mentioned aqueous lubricating coating film is likely to become a thin film. When using the zirconium coating film alone, defects occur in the film and serve as a starting point of corrosion and workability is not obtained, leading to poor corrosion resistance.

As mentioned above, it is possible to realize high workability and high corrosion resistance in a service environment, which have never been achieved by the prior art, by using a coating film of the silicate and the tungstate in combination with a zirconium coating film.

In the present invention, it is suitable that the silicon is derived from a water-soluble silicate, and the tungsten is derived from a water-soluble tungstate.

The water-soluble silicate includes, for example, sodium silicate, potassium silicate, lithium silicate and ammonium silicate. These water-soluble silicates may be used alone, or two or more water-soluble silicates may be used in combination.

The water-soluble tungstate includes, for example, sodium tungstate, potassium tungstate, lithium tungstate and ammonium tungstate. These water-soluble tungstates may be used alone, or two or more water-soluble tungstates may be used in combination.

A zirconium supply source in a film treatment agent for formation of a zirconium coating film according to the present invention includes, for example, zirconium sulfate, zirconium oxysulfate, ammonium zirconium sulfate, zirconium nitrate, zirconium oxynitrate, ammonium zirconium nitrate, zirconium acetate, zirconium lactate, zirconium chloride, fluoroziroconic acid, fluorozirconium complex salt and the like. These zirconium supply sources may be used alone, or two or more zirconium supply sources may be used in combination.

A resin will be described below. The resin is mixed in the upper layer coating film for the purpose of the binder effect, an improvement in adhesion between a base material and a film, imparting of leveling property by the thickening effect, and stabilization of a dispersion component.

Examples of the resin having such function and property include a vinyl resin, an acrylic resin, an epoxy resin, a urethane resin, a phenol resin, a cellulose derivative, a polymaleic acid and a polyester resin. These resins may be used alone, or two or more resins may be used in combination.

In the present invention, the upper layer coating film contains a resin and a mass ratio of resin/(silicon+tungstate) is preferably 0.01 or more, and more preferably 0.1 or more. The mass ratio is preferably 3.2 or less, and more preferably 2.1 or less. If the mass ratio is less than 0.01, the above effects are not sufficiently exerted. Meanwhile, if the mass ratio exceeds 3.2, the amounts of silicon and tungsten relatively decrease, thus failing to exhibit high corrosion resistance and workability which are features of the present invention.

A lubricant will be described below. The lubricant itself has slipperiness, and has a function of reducing a friction force. In general, if the friction force increases during plastic working, an increase in working energy, heat generation, seizure and the like occurs. If the lubricant is included in an upper layer coating of the steel wiring rod of the present invention, the lubricant exists in a lubricating coating film in the form of a solid, thus suppressing an increase in friction force. Examples of the lubricant having such function and property include wax, polytetrafluoroethylene, fatty acid soap, fatty acid metal soap, fatty acid amide, molybdenum disulfide, tungsten disulfide, graphite and melamine cyanurate. These lubricants may be used alone, or two or more lubricants may be used in combination.

Specific examples of the wax include polyethylene wax, paraffin wax, microcrystalline wax, polypropylene wax and carnauba wax. Specific examples of the fatty acid soap include sodium myristate, potassium myristate, sodium palmitate, potassium palmitate, sodium stearate and potassium stearate. Specific examples of the fatty acid metal soap include calcium stearate, zinc stearate, barium stearate, magnesium stearate and lithium stearate. The fatty acid amide is, for example, an amide compound having two fatty acids, and specific examples thereof include ethylenebislauric acid amide, ethylenebisstearic acid amide, ethylenebisbehenic acid amide, N,N′-distearyladipic acid amide, ethylenebisoleic acid amide, ethylenebiserucic acid amide, hexamethylenebisoleic acid amide and N,N′-dioleyladipic acid amide.

In the present invention, when the lubricant is contained in the upper layer coating film, the mass ratio of lubricant/(silicon+tungsten) is preferably 0.01 or more, and more preferably 0.1 or more, and the mass ratio is preferably 3.2 or less, and more preferably 2.1 or less. Here, if the mass ratio of lubricant/(silicon+tungsten) is less than 0.01, it is impossible to exhibit performances because of too small amount of the lubricant. If the mass ratio exceeds 3.2, the amounts of silicon+tungsten relatively decrease, thus failing to exhibit high corrosion resistance and workability which are features of the present invention.

The upper layer coating film of the steel wire rod of the present invention can be mixed with a viscosity modifier, in addition to silicon, tungsten, the resin and the lubricant, for the purpose of imparting leveling property and thixotrophy so as to ensure a uniform coating state when a lubricating treatment solution is applied to a base material. Specific examples of the viscosity modifier include smectite-based clay minerals such as montmorillonite, sauconite, beidellite, hectorite, nontronite, saponite, iron-rich saponite and stevensite; and inorganic thickeners such as pulverized silica, bentonite and kaolin.

The upper layer coating film may contain water-soluble salts, for example, inorganic salts, such as sulfates and borates, and organic salts so as to improve adhesion and workability. Examples of the sulfate include sodium sulfate, potassium sulfate, and the like. Examples of the borate include sodium metaborate, potassium metaborate, ammonium metaborate, and the like.

Examples of the organic salt include salts of formic acid, acetic acid, butyric acid, oxalic acid, succinic acid, lactic acid, ascorbic acid, tartaric acid, citric acid, malic acid, malonic acid, maleic acid, phthalic acid, and the like, with alkali metals, alkali earth metals, and the like.

The coating film of the steel wire rod of the present invention can be imparted with high corrosion resistance before and after working, and may be mixed with other water-soluble rust preventives and inhibitors for the purpose of further improving corrosion resistance. Specifically, it is possible to use known rust preventives and inhibitors, for example, various organic acids such as oleic acid, dimer acid, tartaric acid and citric acid; various chelating agents such as EDTA, NTA, HEDTA and DTPA; mixed components of alkanolamine such as triethanolamine, and amine salts of p-t-butylbenzoic acid; and combinations of a carboxylic acid amine salt, a dibasic acid amine salt, an alkenylsuccinic acid and a water-soluble salt thereof with aminotetrazole and a water-soluble salt thereof. These rust preventives and inhibitors may be used alone, or two or more rust preventives and inhibitors may be used in combination.

The upper layer coating agent used in the present invention contains the water-soluble silicate and the water-soluble tungstate as essential components, and optionally contains the above-mentioned resin, lubricant and water-soluble salts.

The amount of the water-soluble silicate preferably exceeds 5% by mass, more preferably 10% by mass or more, and still more preferably 15% by mass or more, and is also preferably 58% by mass or less, more preferably 52% by mass or less, and still more preferably 45% by mass or less, in 100% by mass of the upper layer coating agent.

The amount of the water-soluble tungstate is preferably 10% by mass or more, more preferably 15% by mass or more, and still more preferably 20% by mass or more, and is also preferably 91% by mass or less, more preferably 85% by mass or less, and still more preferably 80% by mass or less, in 100% by mass of the upper layer coating agent.

If the amount of the water-soluble silicate is 5% by mass or less and the amount of the water-soluble tungstate exceeds 91% by mass, the obtained film cannot achieve sufficient long-term rust prevention property, and is inferior in wire drawability and ball ironing property. This is because the amount of the water-soluble silicate relatively decreases, thus making it easier to transmit moisture, while crystals of tungsten are precipitated, thus degrading adhesion and uniformity of the coating film. If the amount of the water-soluble silicate exceeds 58% by mass and the amount of the water-soluble tungstate is less than 10% by mass, the obtained film can achieve neither sufficient corrosion resistance nor workability. This is because the amount of tungsten relatively decreases, thus failing to sufficiently form a passive film, while the amount of silicate relatively increases to form a firm network structure.

In the lower layer of the steel wire rod of the present invention, the lubricating coating film can be used as a lubricating undercoating film for dry lubricant. Use as an undercoating film of a dry lubricant enables leveling up of lubricity, seize resistance and corrosion resistance. There is no limitation on the dry lubricant and it is possible to use, for example, a general lubricating powder or wire drawing powder which contains, as main components, higher fatty acid soap, borax, lime, molybdenum disulfide, and the like.

In the present invention, a liquid medium (solvent, dispersion medium) in an upper layer coating agent and a zirconium coating agent is water. To shorten the drying time of the lubricant in the drying step, it is possible to mix the upper layer coating agent with an alcohol having a boiling point lower than that of water.

To enhance stability of the solution, the upper layer coating agent may contain a water-soluble strong alkali component. Specific examples thereof include lithium hydroxide, sodium hydroxide and potassium hydroxide. These water-soluble strong alkali components may be used alone, or two or more water-soluble strong alkali components may be used in combination. The amount of these water-soluble strong alkali components is preferably in a range of 0.01 to 10% by mass based on the mass of total solid component.

A method for producing a steel wire rod according to the present invention will be described below. The method according to the present invention includes a cleaning step of a production step of an upper layer coating agent and a lower layer coating agent (aqueous chemical conversion treatment agent) as aqueous lubricating coating agents, and a drying step. Each step will be described below.

<Cleaning Step (Pretreatment Step)>

Before formation of a coating film on a steel wire rod, at least one cleaning treatment selected from shot blasting, sand blasting, wet blasting, peeling, alkali degreasing and pickling is preferably performed. Cleaning as used herein is performed for the purpose of removing oxide scales grown by annealing and the like, and various contaminations (oils, etc.).

<Formation Step of Zirconium Coating Film>

The lower layer coating film is formed by bringing an aqueous chemical conversion treatment solution, which has a pH in a range of 2.5 to 5.0 and contains a water-soluble zirconium compound dissolved therein, into contact with a surface of a steel wire rod to form a lower layer coating film. The aqueous chemical conversion treatment solution may be a solution containing the zirconium supply source. The pH is preferably in a range of 2.8 to 4.8, more preferably 3.1 to 4.6, and still more preferably 3.4 to 4.4. The pH of lower than 2.5 leads to excessive etching, thus causing not only degradation of precipitation efficiency of the coating film, but also disturbing of uniform formation of the coating film. The off of higher than 5.0 leads to degradation of liquid stability and precipitation of a large amount of a zirconium compound or sludge, thus exerting an adverse influence on formation of the coating film.

The aqueous chemical conversion treatment solution may be a commercially available zirconium chemical conversion treatment agent, and the zirconium concentration (on a mass basis) is preferably 10 ppm or more, and more preferably 30 ppm or more, and is also preferably 500 ppm or less, and more preferably 300 ppm or less.

In the present invention, the contact method for forming a zirconium coating film is not particularly limited and includes, for example, a spray treatment, an immersion treatment, pouring treatment, and the like. To promote formation of the coating film, the temperature of the aqueous chemical conversion treatment solution in the case of contacting, namely, the treatment temperature is preferably in a range of 20 to 60° C., and more preferably 30 to 50° C. The contacting time varies depending on the material and structure of a steel wire rod, the concentration of a chemical conversion treatment solution, and the treatment temperature, and is preferably in a rage of approximately 2 to 600 seconds, and can be appropriately adjusted according to the amount of the coating film. After the chemical conversion treatment, a water rinsing step is preferably provided so as not to incorporate the aqueous chemical conversion treatment solution adhered to a steel material into the upper layer coating solution.

<Formation Step of Upper Layer Coating Film>

Next, an upper layer coating film is formed on the zirconium coating film obtained as mentioned above. There no particular limitation on the step of applying the upper layer coating film on a steel wire rod, and it is possible to use coating methods such as an immersion method, a flow coating method and a spraying method. A surface of the steel wire rod is sufficiently coated with an upper layer coating agent as an aqueous lubricating coating agent, and also the coating time is not particularly limited. To enhance drying property in this case, the steel wire rod may be brought into contact with the aqueous lubricating coating agent after heating to a temperature in a range of 60 to 80° C. The steel wire rod may also be brought into contact with the aqueous lubricating coating agent heated to a temperature in a range of 40 to 70° C. Whereby, the drying property may be sometimes improved significantly, thus enabling drying at a normal temperature and reduction in thermal energy loss.

<Drying Step>

There is a need to dry the upper layer coating agent. Drying may be performed by being left to stand at a normal temperature, or may performed at 60 to 150° C. for 1 to 30 minutes.

The mass of an upper layer coating film formed over a steel wire rod is appropriately controlled depending on the degree of subsequent working, and the mass of the coating film is preferably 1.0 g/m² or more, more preferably 2.0 g/m² or more, and is also preferably 20 g/m² or less, and more preferably 15 g/m² or less. Low mass of the coating film leads to insufficient lubricity. It is not preferred that the mass of the coating film exceeds 20 g/m² since clogging occurs in a die, although there is no problem in lubricity. The mass of the coating film can be calculated from a difference in mass between steel wire rods before and after a treatment, and a surface area. To control so as to adjust in a range of the above-mentioned mass of the coating film, the solid component mass (concentration) of the aqueous lubricating coating agent is appropriately adjusted. In practice, after diluting a high concentration lubricant with water, the thus obtained treatment solution is often used. There is no particular limitation on water used for dilution and adjustment, and, for example, pure water, deionized water, tap water, ground water, industrial water, and the like can be used.

<Film Removal Method>

In the present invention, film removal can be performed by immersing the upper layer coating film in an aqueous alkali cleaner, or spraying the aqueous alkali cleaner. The alkali cleaner is a solution prepared by dissolving a common alkali component such as sodium hydroxide or potassium hydroxide in water, and when the alkali cleaner is brought into contact with the upper layer coating film, the upper layer coating film dissolves in the cleaning solution, thus making it possible to easily perform film removal. The coating film thus obtained easily falls off by a heat treatment after working. Therefore, alkali cleaning enables prevention of contamination and poor plating in the subsequent step caused by insufficient film removal.

A method for analysis of the composition of a coating film will be described below.

Examples of the method for analysis of a zirconium coating film include a method of observation using a cross-sectional scanning electron microscope (SEM), or a method in which a film thickness is directly observed in observation using a cross-sectional transmission electron microscope (TEM). At this time, it is effective to expose a cross-section by working using a cross-section polisher (CP) or working using focused ion beams (FIB) so as to perform cross-sectional observation without damaging a coating film. A simple method includes a method in which a thickness of a zirconium coating film on a surface of a steel material is analyzed by fluorescent X-ray spectroscopy (X-ray fluorescence (XRF)). At this time, an upper layer coating film is preferably peeled with an aqueous alkali solution. A film thickness of the lower layer coating film in the following manner: three regions where a film thickness is measured are selected at random on a surface of a steel material after peeling the upper layer coating film, and then the film thickness is measured in all of selected regions. An average of thicknesses measured in three regions is regarded as a film thickness of an undercoating film of the steel material.

The method for analysis of the composition of the upper layer coating film includes, for example, a method using inductively coupled plasma (ICP). In this case, the coating film on the steel material is dissolved in an aqueous strong alkali solution and the amounts of silicon and tungsten dissolved are measured by ICP, thus enabling analysis of the composition of the coating film.

When it is difficult to dissolve the upper layer coating film, it is also possible to analyze the composition of the coating film by using a method in which the amounts of silicon and tungsten on a surface of a steel wire rod by fluorescent X-ray spectroscopy (X-ray fluorescence (XRF)).

EXAMPLES

The present invention will be described below in a more specific manner by way of Examples and Comparative Examples, together with effects thereof, with respect to a steel wire rod. The present invention is not limited to these Examples. In the following description, parts are by mass and percentages are by mass, unless otherwise specified.

(1-1) Preparation of Upper Layer Coating Agent as Aqueous Lubricating Coating Agent and Lower Layer Coating Agent

In accordance with the combination and proportion shown in Table 1, upper layer coating agents as aqueous lubricating coating agents and lower layer coating agents of Examples 1 to 12 and Comparative Examples 2 to 17 were prepared using the respective components shown below. To upper layer coating agents of Examples 1 to 12 and Comparative Examples 3 to 17, lithium hydroxide was added in the concentration of 1% in the solution so as to enhance liquid stability. Comparative Example 18 means the case subjected to a phosphate/soap treatment, and a wire drawing powder was not used.

A. Upper Layer Coating Agent <Water-Soluble Silicate>

-   (A-1) Sodium metasilicate -   (A-2) JIS No. 3 sodium silicate (Na₂O·nSiO₂, n=3) -   (A-3) Lithium silicate (Li₂O·nSiO₂, n=3.5)

<Water-Soluble Tungstate>

-   (B-1) Ammonium tungstate -   (B-2) Sodium tungstate -   (B-3) Potassium tungstate

<Resin>

-   (C-1) Polyvinyl alcohol (average molecular weight of about 50,000) -   (C-2) Sodium neutralizing salt of isobutylene-maleic anhydride     copolymer (average molecular weight of about 165,000)

<Lubricant>

-   (D-1) Polyethylene wax (average particle size of 5 μm) -   (D-2) Ethylenebis-stearic acid amide

<Water-Soluble Salt>

-   (E-1) Sodium metaborate -   (E-2) Sodium tartrate -   (E-3) Sodium sulfate -   (E-4) Sodium Pyrophosphate

B. Lower Layer Coating Agent <Zirconium Lower Layer Coating Film>

-   (F) Zirconium chemical conversion treatment agent (PALLUCID     (registered trademark) 1500, manufactured by Nihon Parkerizing Co.,     Ltd.)     <Lower Layer Coating Film except for Zirconium Lower Layer Coating     Film> -   (G-1) JIS No. 2 sodium silicate (Na₂O·nSiO₂, n=2.5) -   (G-2) Zinc phosphate

TABLE 1 Water-soluble silicate Water-soluble tungstate Zirconium Resin Lubricant Water-soluble salt Component (A) Component (B) coating Component (C) Component (D) Component (E) (A-1) (A-2) (A-3) (B-1) (B-2) (B-3) film (nm) (C-1) (C-2) (D-1) (D-2) (E-1) (E-2) Example 1 20 20 0 30 30 0 55 0 0 0 0 0 0 Example 2 0 15 10 15 60 0 195 0 0 0 0 0 0 Example 3 0 10 0 59 0 20 65 11 0 0 0 0 0 Example 4 0 0 40 0 0 40 75 0 5 15 0 0 0 Example 5 30 0 0 50 0 0 1 0 10 0 10 0 0 Example 6 40 0 10 0 40 0 150 7 0 0 3 0 0 Example 7 0 40 0 0 25 30 10 0 0 5 0 0 0 Example 8 10 0 5 17 0 10 22 18 0 20 20 0 0 Example 9 10 5 0 0 20 0 33 0 15 45 5 0 0 Example 10 0 10 10 10 10 0 55 15 25 0 20 0 0 Example 11 11 0 0 0 10 12 80 7 40 15 5 0 0 Example 12 0 15 0 0 0 15 5 0 35 35 0 0 0 Comparative No coating film (only dry lubricant) Example 1 Comparative 0 0 0 0 0 0 100 0 0 0 0 0 0 Example 2 Comparative 25 0 15 0 0 60 0.6 0 0 0 0 0 0 Example 3 Comparative 0 30 0 0 70 0 230 0 0 0 0 0 0 Example 4 Comparative 75 0 0 25 0 0 80 0 0 0 0 0 0 Example 5 Comparative 0 10 0 90 0 0 66 0 0 0 0 0 0 Example 6 Comparative 0 100 0 0 0 0 100 0 0 0 0 0 0 Example 7 Comparative 0 0 0 0 0 90 90 10 0 0 0 0 0 Example 8 Comparative 0 0 0 60 0 0 95 0 15 0 0 25 0 Example 9 Comparative 0 0 0 0 35 30 85 10 0 0 0 0 25 Example 10 Comparative 50 15 0 0 0 0 60 0 0 0 0 0 10 Example 11 Comparative 0 55 15 0 0 0 128 0 0 0 0 0 0 Example 12 Comparative 0 0 0 0 0 0 51 0 20 0 0 40 0 Example 13 Comparative 0 0 0 0 0 0 111 5 0 0 0 95 0 Example 14 Comparative 0 0 0 0 0 0 50 0 10 0 0 0 90 Example 15 Comparative 35 0 0 35 20 0 0 0 10 0 0 0 0 Example 16 Comparative 0 10 10 10 70 0 0 0 0 0 0 0 0 Example 17 Comparative Phosphate/soap treatment Example 18 Mass of coating film per unit Mass ratio Mass ratio Water-soluble salt Undercoating area of upper Mass ratio of resin/ of lubricant/ Component (E) film (mg/m²) layer coating of tungsten/ (silicon + (silicon + (E-3) (E-4) (G-1) (G-2) film (g/m²) silicon tungsten) tungsten) Example 1 0 0 0 0 7.5 3.6 — — Example 2 0 0 0 0 4.1 5.2 — — Example 3 0 0 0 0 6.6 16.1  0.19 — Example 4 0 0 0 0 7.7 1.4 0.13 0.39 Example 5 0 0 0 0 1.6 5.4 0.23 0.23 Example 6 0 0 0 0 14.0 1.9 0.18 0.08 Example 7 0 0 0 0 3.5 2.5 — 0.11 Example 8 0 0 0 0 5.0 4.5 0.79 1.76 Example 9 0 0 0 0 3.0 2.7 0.91 3.02 Example 10 0 0 0 0 13.1 1.8 1.88 0.94 Example 11 0 0 0 0 18.9 5.0 3.02 1.29 Example 12 0 0 0 0 9.9 1.5 2.56 2.56 Comparative No coating film (only dry lubricant) Example 1 Comparative 0 0 0 0 — — — — Example 2 Comparative 0 0 0 0 6.5 2.9 — — Example 3 Comparative 0 0 0 0 5.5 4.1 — — Example 4 Comparative 0 0 0 0 3.3 1.1 — — Example 5 Comparative 0 0 0 0 2.2 19.6  — — Example 6 Comparative 0 0 0 0 2.8 — — — Example 7 Comparative 0 0 0 0 4.4 — — — Example 8 Comparative 0 0 0 0 9.9 — 0.34 — Example 9 Comparative 0 0 0 0 10.1 — 0.26 — Example 10 Comparative 25 0 0 0 8.1 — — — Example 11 Comparative 0 30 0 0 7.7 — — — Example 12 Comparative 0 40 0 0 — — — — Example 13 Comparative 0 0 0 0 — — — — Example 14 Comparative 0 0 0 0 — — — — Example 15 Comparative 0 0 1.9 0 8.9 4.9 0.21 — Example 16 Comparative 0 0 0 4.3 7.4 6.6 — — Example 17 Comparative Phosphate/soap treatment Example 18 *Numerical in each of components (A) to (E) means an addition amount in an upper layer coating agent (% by mass).

(1-2) Analysis of Coating Film

A test material in size of φ3.2 mm×1 m subjected to a treatment for formation of a lower layer coating film and an upper layer coating film was immersed in an aqueous 2% sodium hydroxide solution heated at 60° C. for 2 minutes and the upper layer coating film was peeled. Thereafter, the amounts of silicon and tungsten contained in the aqueous sodium hydroxide solution used for peeling were measured by inductively coupled plasma (ICP) and the value of a mass ratio of tungsten/silicon was examined. After immersing in an aqueous 2% sodium hydroxide solution heated at 60° C. for 2 minutes, the excessive upper layer coating film was peeled. A film thickness of a zirconium coating film was measured by subjecting the test material whose upper layer coating was peeled to fluorescent X-ray spectroscopy (XRF). The measured value is shown in Table 1. By carrying out cross-sectional SEM analysis, the thickness of the lower layer coating film was measured and the film thickness was measured. In the measurement of the film thickness, the film thickness of the lower layer coating film (namely, zirconium coating film) was measured in the following manner: three regions where a film thickness is measured are selected at random on a surface of a steel material after peeling the upper layer coating film, and then the film thickness is measured in all of selected regions. An average of thicknesses measured in three regions is regarded as a film thickness of an undercoating film of the steel material.

(1-3) Coating Treatment

A coating treatment method is shown below. The material to be treated is a φ3.2 mm steel wire rod. To reproduce thinning of a lubricating coating film in the binding portion, the treatment was performed in a state of being bundled with a binding band made of a plastic.

Pretreatment and Coating Treatment of Examples 1 to 12 and Comparative Examples 3 to 15

-   (a) Degreasing: commercially available degreasing agent (FINECLEANER     (registered trademark) 6400, manufactured by Nihon Parkerizing Co.,     Ltd.), concentration: 20 g/L, temperature: 60° C., immersion: 10     minutes -   (b) Water rinsing: tap water, normal temperature, immersion: 30     seconds -   (c) Pickling: hydrochloric acid, concentration: 17.5%, normal     temperature, immersion: 10 minutes -   (d) Water rinsing: tap water, normal temperature, immersion: 30     seconds -   (e) Lower layer coating treatment: commercially available zirconium     chemical conversion treatment agent (PALLUCID (registered trademark)     1500, manufactured by Nihon Parkerizing Co., Ltd.), concentration:     50 g/L, temperature: 45° C., pH 4.0, immersion treatment: immersion     time is appropriately adjusted according to the amount of the     coating film. -   (f) Water rinsing: tap water, normal temperature, immersion: 30     seconds -   (g) Upper layer coating treatment: upper layer coating agent     prepared in (1-1), temperature: 60° C., immersion: 1 minute -   (h) Drying: 100° C., 10 minutes

Pretreatment and Coating Treatment of Comparative Example 1

-   (a) Degreasing: commercially available degreasing agent (FINECLEANER     (registered trademark) 6400, manufactured by Nihon Parkerizing Co.,     Ltd.), concentration: 20 g/L, temperature: 60° C., immersion: 10     minutes -   (b) Water rinsing: tap water, normal temperature, immersion: 30     seconds -   (c) Pickling: hydrochloric acid, concentration: 17.5%, normal     temperature, immersion: 10 minutes -   (d) Water rinsing: tap water, normal temperature, immersion: 30     seconds -   (e) Pure water rinsing: deionized water, normal temperature,     immersion: 30° C. -   (f) Drying: 100° C., 10 minutes

Pretreatment and Coating Treatment of Comparative Example 2

-   (a) Degreasing: commercially available degreasing agent (FINECLEANER     (registered trademark) 6400, manufactured by Nihon Parkerizing Co.,     Ltd.), concentration: 20 g/L, temperature: 60° C., immersion: 10     minutes -   (b) Water rinsing: tap water, normal temperature, immersion: 30     seconds -   (c) Pickling: hydrochloric acid, concentration: 17.5%, normal     temperature, immersion: 10 minutes -   (d) Water rinsing: tap water, normal temperature, immersion: 30     seconds -   (e) Lower layer coating treatment: commercially available zirconium     chemical conversion treatment agent (PAILUCID (registered trademark)     1500, manufactured by Nihon Parkerizing Co., Ltd.), concentration:     50 g/L, temperature: 45° C., pH 4.0, immersion treatment: immersion     time is appropriately adjusted according to the amount of the     coating film. -   (f) Water rinsing: tap water, normal temperature, immersion: 30° C. -   (g) Pure water rinsing: deionized water, normal temperature,     immersion: 30° C. -   (h) Drying: 100° C., 10 minutes

Comparative Example 16 (Silicate Lower Layer Coating Film, Upper Layer Coating Treatment)

-   (a) Degreasing: commercially available degreasing agent (FINECLEANER     (registered trademark) 6400, manufactured by Nihon Parkerizing Co.,     Ltd.), concentration: 20 g/L, temperature: 60° C., immersion: 10     minutes -   (b) Water rinsing: tap water, normal temperature, immersion: 30     seconds -   (c) Pickling: hydrochloric acid, concentration: 17.5%, normal     temperature, immersion: 10 minutes -   (d) Water rinsing: tap water, normal temperature, immersion: 30     seconds -   (e) Lower layer coating treatment: commercially available surface     treatment agent (PREPALENE (registered trademark) 5557, manufactured     by Nihon Parkerizing Co., Ltd.), concentration 2.5 g/L, temperature     70° C., immersion: 1 minute -   (f) Rough drying: normal temperature, 60 seconds -   (g) Upper layer coating treatment: upper layer coating agent     prepared in (1-1), temperature: 60° C., immersion: 1 minute -   (h) Drying: 100° C., 10 minutes

Pretreatment and Coating Treatment of Comparative Example 17 (Zinc Phosphate Lower Layer Coating Film, Upper Layer Coating Treatment)

-   (a) Degreasing: commercially available degreasing agent (FINECLEANER     (registered trademark) 6400, manufactured by Nihon Parkerizing Co.,     Ltd.), concentration: 20 g/L, temperature: 60° C., immersion: 10     minutes -   (b) Water rinsing: tap water, normal temperature, immersion: 30     seconds -   (c) Pickling: hydrochloric acid, concentration: 17.5%, normal     temperature, immersion: 10 minutes -   (d) Water rinsing: tap water, normal temperature, immersion: 30     seconds -   (e) Lower layer coating treatment: commercially available zinc     phosphate chemical conversion treatment agent (PALBOND (registered     trademark) 3696X, manufactured by Nihon Parkerizing Co., Ltd.),     concentration: 75 g/L, temperature 80° C., immersion: 10 minutes -   (f) Water rinsing: tap water, normal temperature, immersion: 30     seconds -   (g) Upper layer coating treatment: upper layer coating agent     prepared in (1-1), temperature: 60° C., immersion: 1 minute -   (h) Drying: 100° C., 10 minutes

Pretreatment and Coating Treatment of Comparative Example 18 (Phosphate/Soap Treatment)

-   (a) Degreasing: commercially available degreasing agent (FINECLEANER     (registered trademark) 6400, manufactured by Nihon Parkerizing Co.,     Ltd.), concentration: 20 g/L, temperature: 60° C., immersion: 10     minutes -   (b) Water rinsing: tap water, normal temperature, immersion: 30     seconds -   (c) Pickling: hydrochloric acid, concentration: 17.5%, normal     temperature, immersion: 10 minutes -   (d) Water rinsing: tap water, normal temperature, immersion: 30     seconds -   (e) Chemical conversion coating: commercially available zinc     phosphate chemical conversion treatment agent (PALBOND (registered     trademark) 3696X, manufactured by Nihon Parkerizing Co., Ltd.),     concentration: 75 g/L, temperature 80° C., immersion: 10 minutes -   (f) Water rinsing: tap water, normal temperature, immersion: 30     seconds -   (g) Soap treatment: commercially available reactive soap lubricant     (PALUBE (registered trademark) 235, manufactured by Nihon     Parkerizing Co., Ltd.), concentration: 70 g/L, temperature 85° C.,     immersion: 3 minutes -   (h) Drying: 100° C., 10 minutes -   (i) Amount of dry coating film: 10 g/m²

(1-4) Evaluation Test (1-4-1) Workability (Wire Drawability) Test

Wire drawing was performed by drawing a sample wire rod in size of φ3.2 mm×20 m through a φ2.76 die. Missile C40 available from Matsuura Kougyo K. K. was used as a dry lubricant. At the position immediately before drawing a material, a die box with the dry lubricant was provided so that the dry lubricant naturally adheres to the material. Evaluation was made from seizure of the test material and the remaining amount of the lubricating coating film after wire drawing.

Evaluation Criteria

-   A: No seizure occurs and no metal gloss is recognized and, on the     whole, a film remains in a large amount. -   B: No seizure occurs and no metal gloss is recognized, and a film     remains in an amount which slightly smaller than that in A. -   C: No seizure occurs and a film retention amount is slightly small,     and metal gloss is partially recognized. -   D: No seizure occurs and metal gloss is recognized at numerous     sites. -   E: Seizure occurred.

(1-4-2) Corrosion Resistance (Long-Term Rust Prevention Property) Test

In summer season, a wire rod subjected to the above-mentioned wire drawing test was exposed to an open air atmosphere indoors for two weeks or four months, and then the degree of rusting was observed. It was judged that the more the rust area increases, the more corrosion resistance becomes inferior.

Evaluation Criteria

-   A: Extremely excellent as compared with a phosphate/soap coating     film (rust area of less than 5%) -   B: Excellent as compared with a phosphate/soap coating film (rust     area of 5% or more and less than 15%) -   C: Equivalent to a phosphate/soap coating film (rust area of 15% or     more and less than 25%) -   D: Inferior as compared with a phosphate/soap coating film (rust     area of 25% or more and less than 35%) -   E: Drastically inferior as compared with a phosphate/soap coating     film (rust area of 35% or more)

The test results are shown in Table 2. In all Examples, a coating film remains in a large amount, resulting in satisfactory workability and satisfactory corrosion resistance. In Comparative Example 1 in which the same upper and lower layer coating films as those in the present invention are not used, seizure occurred during wire drawing. In Comparative Example 2 in which a test was performed using only a zirconium coating film, workability was insufficient since seizure occurred, like Comparative Example 1, and corrosion resistance was also insufficient. In Comparative Examples 3 and 4, a film thickness of a zirconium coating film used deviates from the scope of the present invention. In Comparative Example 3 in which a film thickness of a zirconium coating film is too small, corrosion resistance degraded, whereas, workability tends to be degraded in Comparative Example 4 in which a film thickness of a zirconium coating film is too large. In Comparative Examples 5 to 12, a mass ratio of silicon to tungsten was set at a value deviating from the scope of the present invention, and workability was inferior and also corrosion resistance was inferior because of small film retention amount after wire drawing. In Comparative Examples 13 to 15 in which components other than a water-soluble silicate and a water-soluble tungstate are included as an aqueous inorganic salt, workability was inferior and also corrosion resistance was inferior because of a small film retention amount after wire drawing. In Comparative Example 16 in which a coating film of a water-soluble silicate was formed as a lower layer coating film, it was impossible to obtain high corrosion resistance comparable to Examples. In Comparative Example 17, a coating film of a phosphate was formed as a lower layer coating film. Although workability and corrosion resistance are in the same level as in Examples, this film is not preferred since it contains phosphorus to cause the above-mentioned problem such as phosphorizing of a bolt. In Comparative Example 18 in which a phosphate coating film was subjected to a reactive soap treatment, like Comparative Example 17, a phosphate coating film is not preferred because of a problem such as phosphorizing of a bolt. Regarding phosphorizing, like Comparative Examples 12 and 13, the same applies to the case of containing a phosphate as a water-soluble salt.

It is possible to impart high corrosion resistance even when an aqueous lubricating coating film became thinner at the bundled portion as a result of bundling materials using a binding band.

TABLE 2 Corrosion Corrosion Workability resistance resistance Phosphatizing (Wire drawability) (two weeks) (four months) property*1 Example 1 A A A B Example 2 A A A B Example 3 B A B B Example 4 A A B B Example 5 A A A B Example 6 A A A B Example 7 A A B B Example 8 A A A B Example 9 B A B B Example 10 B A A B Example 11 B A B B Example 12 B A B B Comparative Example 1 E E E B Comparative Example 2 E E E B Comparative Example 3 A D D B Comparative Example 4 C A A B Comparative Example 5 C B C B Comparative Example 6 D B C B Comparative Example 7 D D D B Comparative Example 8 D D D B Comparative Example 9 D D D B Comparative Example 10 D D D B Comparative Example 11 C E E B Comparative Example 12 C E E E Comparative Example 13 D E E E Comparative Example 14 D E E B Comparative Example 15 E E E B Comparative Example 16 B C C B Comparative Example 17 A B B E Comparative Example 18 B B B E *1B: There is no possibility of brittle fracture of a steel wire rod due to phosphorus because of containing no phosphorus. E: There is possibility of brittle fracture of a steel wire rod due to phosphorus because of containing phosphorus.

The present invention will be described below in a more specific manner by way of Examples and Comparative Examples, together with effects thereof, with respect to a steel wire. The present invention is not limited to these Examples. In the following description, parts are by mass and percentages are by mass, unless otherwise specified.

(2-1) Preparation of Upper Layer Coating Agent and Lower Layer Coating Agent as Aqueous Lubricating Coating Agent

In accordance with the combination and proportion shown in Table 3, upper layer coating agents and lower layer coating agents of Examples 13 to 29 and Comparative Examples 19 to 35 were prepared using the respective components shown below. Comparative Example 36 means the case subjected to a phosphate/soap treatment.

A. Upper Layer Coating Agent <Water-Soluble Silicate>

-   (A-1) Sodium metasilicate -   (A-2) JIS No. 3 sodium silicate (Na₂O·nSiO₂, n=3) -   (A-3) Lithium silicate (Li₂O·nSiO₂, n=3.5)

<Water-Soluble Tungstate>

-   (B-1) Ammonium tungstate -   (B-2) Sodium tungstate -   (B-3) Potassium tungstate

<Resin>

-   (C-1) Polyvinyl alcohol (average molecular weight of about 50, 000) -   (C-2) Sodium neutralizing salt of isobutylene-maleic anhydride     copolymer (average molecular weight of about 165,000) -   (C-3) Carboxymethylcellullose sodium (average molecular weight of     about 30,000) -   (C-4) Aqueous nonionic urethane resin emulsion

<Lubricant>

-   (D-1) Anionic polyethylene wax (average particle size of 5 μm) -   (D-2) Ethylenebis-stearic acid amide -   (D-3) Calcium stearate -   (D-4) Polytetrafluoroethylene dispersion (average particle size of     0.2 μm)

<Water-Soluble Salt>

-   (E-1) Sodium metaborate -   (E-2) Sodium tartrate -   (E-3) Sodium sulfate -   (E-4) Sodium pyrophosphate

B. Lower Layer Coating Agent <Zirconium Lower Layer Coating Film>

-   (F) Zirconium chemical conversion treatment agent (PALLUCID     (registered trademark) 1500, manufactured by Nihon Parkerizing Co.,     Ltd.)     <Lower Layer Coating Film other than Zirconium Lower Layer Coating     Film> -   (G-1) No. 2 sodium silicate (Na₂O·nSiO₂, n=2.5) -   (G-2) Zinc phosphate

TABLE 3 Water-soluble silicate Water-soluble tungstate Zirconium Resin Lubricant Component (A) Component (B) coating Component (C) Component (D) (A-1) (A-2) (A-3) (B-1) (B-2) (B-3) film (nm) (C-1) (C-2) (C-3) (C-4) (D-1) (D-2) (D-3) (D-4) Example 13 0 20 0 40 0 0 99 0 20 0 0 20 0 0 0 Example 14 30 0 0 20 0 0 75 0 40 0 0 10 0 0 0 Example 15 0 8 0 15 51 0 30 13 0 0 0 0 0 13 0 Example 16 10 20 10 20 15 0 87 0 0 0 10 5 10 0 0 Example 17 10 10 0 0 40 10 123 0 0 10 0 0 10 0 10 Example 18 0 30 0 0 0 25 50 0 25 0 0 5 5 0 10 Example 19 0 15 10 0 0 50 150 0 0 0 10 0 0 15 0 Example 20 0 0 22 0 0 55 10 8 0 0 0 10 0 0 5 Example 21 10 0 20 0 20 20 1 0 0 0 15 5 0 0 10 Example 22 0 20 0 10 40 0 200 0 15 0 0 0 0 15 0 Example 23 0 0 12 63 0 0 112 0 0 0 0 0 0 0 25 Example 24 0 40 0 0 38 0 91 15 0 0 0 7 0 0 0 Example 25 0 10 10 0 10 10 80 0 25 0 5 20 10 0 0 Example 26 17 0 0 0 0 21 66 0 37 0 10 10 5 0 0 Example 27 0 20 0 10 5 0 54 13 0 0 0 25 10 0 15 Example 28 10 32 0 15 20 0 22 10 0 0 0 10 0 0 0 Example 29 0 10 0 0 30 0 35 0 20 0 0 25 0 10 5 Comparative 0 0 0 0 0 0 100 0 0 0 0 0 0 0 0 Example 19 Comparative 0 20 0 40 0 0 0.7 0 20 0 0 20 0 0 0 Example 20 Comparative 30 0 0 10 0 40 250 0 0 10 0 0 10 0 0 Example 21 Comparative 50 0 0 0 0 0 82 0 10 20 0 0 0 20 0 Example 22 Comparative 0 0 0 60 0 0 73 20 0 0 0 0 20 0 0 Example 23 Comparative 0 0 30 0 0 20 51 0 0 0 15 35 0 0 0 Example 24 Comparative 6 0 0 15 30 0 30 0 19 0 15 0 0 0 15 Example 25 Comparative 45 0 0 0 0 0 120 0 0 15 0 0 0 15 0 Example 26 Comparative 0 20 20 0 0 0 31 10 0 0 20 0 0 10 10 Example 27 Comparative 0 0 0 40 0 0 20 0 15 0 0 20 0 0 0 Example 28 Comparative 0 0 0 0 50 0 77 10 0 5 0 5 0 10 0 Example 29 Comparative 0 0 0 0 0 0 111 0 5 0 0 0 10 0 25 Example 30 Comparative 0 0 0 0 0 0 93 10 0 0 10 0 20 0 0 Example 31 Comparative 0 0 0 0 0 0 91 0 0 25 0 0 25 0 0 Example 32 Comparative 0 0 0 0 0 0 88 0 25 0 0 15 0 20 0 Example 33 Comparative 0 10 10 0 0 45 0 15 0 0 0 15 0 0 5 Example 34 Comparative 30 0 0 0 50 0 0 0 0 10 0 10 0 0 0 Example 35 Comparative Phosphate/soap treatment Example 36 Mass of coating film per unit Mass ratio Mass ratio Water-soluble alt Undercoating area of upper Mass ratio of resin/ of lubricant/ Component (E) film (mg/m²) layer coating of tungsten/ (silicon + (silicon + (E-1) (E-2) (E-3) (E-4) (G-1) (G-2) film (g/m²) silicon tungsten) tungsten) Example 13 0 0 0 0 0 0 10.1 4.4 0.54 0.54 Example 14 0 0 0 0 0 0 13.1 2.5 1.83 0.46 Example 15 0 0 0 0 0 0 12.2 16.0  0.28 0.28 Example 16 0 0 0 0 0 0 1.4 1.9 0.27 0.40 Example 17 0 0 0 0 0 0 8.1 5.3 0.27 0.55 Example 18 0 0 0 0 0 0 5.2 1.5 1.02 0.82 Example 19 0 0 0 0 0 0 4.1 3.0 0.27 0.40 Example 20 0 0 0 0 0 0 7.7 3.4 0.20 0.37 Example 21 0 0 0 0 0 0 13.8 2.2 0.44 0.44 Example 22 0 0 0 0 0 0 7.6 4.7 0.38 0.38 Example 23 0 0 0 0 0 0 6.6 9.6 — 0.48 Example 24 0 0 0 0 0 0 9.0 1.7 0.40 0.19 Example 25 0 0 0 0 0 0 9.0 1.8 1.54 1.54 Example 26 0 0 0 0 0 0 8.1 2.7 2.98 0.95 Example 27 2 0 0 0 0 0 4.4 1.5 0.74 2.85 Example 28 0 3 0 0 0 0 6.6 1.8 0.27 0.27 Example 29 0 0 0 0 0 0 17.7 5.4 0.90 1.80 Comparative 0 0 0 0 0 0 — — — — Example 19 Comparative 0 0 0 0 0 0 8.2 4.3 0.54 0.54 Example 20 Comparative 0 0 0 0 0 0 6.6 4.3 0.27 0.27 Example 21 Comparative 0 0 0 0 0 0 11.1 — 2.61 1.74 Example 22 Comparative 0 0 0 0 0 0 12.2 — 0.45 0.45 Example 23 Comparative 0 0 0 0 0 0 4.2 0.9 0.64 1.49 Example 24 Comparative 0 0 0 0 0 0 5.1 21.7  1.08 0.48 Example 25 Comparative 0 0 25 0 0 0 9.9 — 1.45 1.45 Example 26 Comparative 0 0 0 10 0 0 10.2 — 1.98 1.32 Example 27 Comparative 25 0 0 0 0 0 8.8 — 0.50 0.67 Example 28 Comparative 10 10 0 0 0 0 7.2 — 0.48 0.48 Example 29 Comparative 30 0 0 30 0 0 — — — — Example 30 Comparative 0 40 20 0 0 0 — — — — Example 31 Comparative 50 0 0 0 0 0 — — — — Example 32 Comparative 0 40 0 0 0 0 — — — — Example 33 Comparative 0 0 0 0 1.5 0 4.6 3.4 0.46 0.61 Example 34 Comparative 0 0 0 0 0 4.4 5.8 4.5 0.26 0.26 Example 35 Comparative Phosphate/soap treatment Example 36 *Numerical in each of components (A) to (E) means an addition amount in an upper layer coating agent (% by mass).

(2-2) Analysis of Coating Film

An SPCC-SD material was subjected to a treatment for formation of a lower layer coating film and an upper layer coating film, and then the amounts of silicon and tungsten on a surface were directly measured by fluorescent X-ray spectroscopy (XRF) and the value of a mass ratio of tungsten/silicon was examined. After immersing in an aqueous 2% sodium hydroxide solution heated at 60° C. for 2 minutes, the upper layer coating film was peeled. A film thickness of a zirconium coating film was measured by subjecting the test material whose upper layer coating was peeled to fluorescent X-ray spectroscopy (XRF). The measured values are shown in Table 3. The film thickness measured from XRF is consistent with the results obtained by directly measuring the thickness of a zirconium coating film by carrying out cross-sectional SEM analysis.

(2-3) Coating Treatment Pretreatment and Coating Treatment of Examples 13 to 29 and Comparative Examples 20 to 33

-   (a) Degreasing: commercially available degreasing agent (FINECLEANER     (registered trademark) E6400, manufactured by Nihon Parkerizing Co.,     Ltd.), concentration: 20 g/L, temperature: 60° C., immersion: 10     minutes -   (b) Water rinsing: tap water, normal temperature, immersion: 20     seconds -   (c) Pickling: 17.5% hydrochloric acid, normal temperature,     immersion: 20 minutes -   (d) Water rinsing: tap water, normal temperature, immersion: 20     seconds -   (e) Lower layer coating treatment: commercially available zirconium     chemical conversion treatment agent (PALLUCID (registered trademark)     1500, manufactured by Nihon Parkerizing Co., Ltd.), concentration:     50 g/L, temperature: 45° C., pH 4.0, immersion treatment: immersion     time is appropriately adjusted according to the amount of the     coating film. -   (f) Water rinsing: tap water, normal temperature, immersion: 20     seconds -   (g) Neutralization: commercially available neutralizer (PREPALENE     (registered trademark) 27, manufactured by Nihon Parkerizing Co.,     Ltd.) -   (h) Upper layer coating film treatment: upper layer coating agent     prepared in (2-1), temperature: 60° C., immersion: 1 minute -   (i) Drying: 100° C., 10 minutes

Pretreatment and Coating Treatment of Comparative Example 19

-   (a) Degreasing: commercially available degreasing agent (FINECLEANER     (registered trademark) 6400, manufactured by Nihon Parkerizing Co.,     Ltd.), concentration: 20 g/L, temperature: 60° C., immersion: 10     minutes -   (b) Water rinsing: tap water, normal temperature, immersion: 30     seconds -   (c) Pickling: hydrochloric acid concentration: 17.5%, normal     temperature, immersion: 10 minutes -   (d) Water rinsing: tap water, normal temperature, immersion: 30     seconds -   (e) Lower layer coating treatment: commercially available zirconium     chemical conversion treatment agent (PALLUCID (registered trademark)     1500, manufactured by Nihon Parkerizing Co., Ltd.), concentration:     50 g/L, temperature: 45° C., pH 4.0, immersion treatment: immersion     time is appropriately adjusted according to the amount of the     coating film. -   (f) Water rinsing: tap water, normal temperature, immersion: 30° C. -   (g) Pure water rinsing: deionized water, normal temperature,     immersion: 30° C. -   (h) Drying: 100° C., 10 minutes

Comparative Example 34 (Silicate lower Layer Coating Film, Upper Layer Coating Film Treatment)

-   (a) Degreasing: commercially available degreasing agent (FINECLEANER     (registered trademark) 6400, manufactured by Nihon Parkerizing Co.,     Ltd.), concentration: 20 g/L, temperature: 60° C., immersion: 10     minutes -   (b) Water rinsing: tap water, normal temperature, immersion: 30     seconds -   (c) Pickling: hydrochloric acid, concentration: 17.5%, normal     temperature, immersion: 10 minutes -   (d) Water rinsing: tap water, normal temperature, immersion: 30     seconds -   (e) Lower layer coating treatment: commercially available surface     treatment agent (PREPALENE (registered trademark) 5557, manufactured     by Nihon Parkerizing Co., Ltd.), concentration 2.5 g/L, temperature     70° C., immersion: 1 minute -   (f) Rough drying: normal temperature, 60 seconds -   (g) Upper layer coating film treatment: upper layer coating agent     prepared in (2-1), temperature: 60° C., immersion: 1 minute -   (h) Drying: 100° C., 10 minutes

Pretreatment and Coating Treatment of Comparative Example 35 (zinc phosphate lower layer coating film, upper layer coating film treatment)

-   (a) Degreasing: commercially available degreasing agent (FINECLEANER     (registered trademark) 6400, manufactured by Nihon Parkerizing Co.,     Ltd.), concentration: 20 g/L, temperature: 60° C., immersion: 10     minutes -   (b) Water rinsing: tap water, normal temperature, immersion: 30     seconds -   (c) Pickling: hydrochloric acid, concentration: 17.5%, normal     temperature, immersion: 10 minutes -   (d) Water rinsing: tap water, normal temperature, immersion: 30     seconds -   (e) Lower layer coating treatment: commercially available zinc     phosphate chemical conversion treatment agent (PALBOND (registered     trademark) 3696X, manufactured by Nihon Parkerizing Co., Ltd.),     concentration: 75 g/L, temperature 80° C., immersion: 10 minutes -   (f) Water rinsing: tap water, normal temperature, immersion: 30     seconds -   (g) Upper layer coating treatment: upper layer coating agent     prepared in (2-1), temperature: 60° C., immersion: 1 minute -   (h) Drying: 100° C., 10 minutes

Pretreatment and Coating Treatment of Comparative Example 36 (Phosphate/Soap Treatment)

-   (a) Degreasing: commercially available degreasing agent (FINECLEANER     (registered trademark) 6400, manufactured by Nihon Parkerizing Co.,     Ltd.), concentration: 20 g/L, temperature: 60° C., immersion: 10     minutes -   (b) Water rinsing: tap water, normal temperature, irrunersion: 30     seconds -   (c) Pickling: hydrochloric acid, concentration: 17.5%, normal     temperature, immersion: 10 minutes -   (d) Water rinsing: tap water, normal temperature, immersion: 30     seconds -   (e) Chemical conversion coating: commercially available zinc     phosphate chemical conversion treatment agent (PALBOND (registered     trademark) 3696X, manufactured by Nihon Parkerizing Co., Ltd.),     concentration: 75 g/L, temperature 80° C., immersion: 7 minutes -   (f) Water rinsing: tap water, normal temperature, immersion: 30     seconds -   (g) Soap treatment: commercially available reactive soap lubricant     (PALUBE (registered trademark) 235, manufactured by Nihon     Parkerizing Co., Ltd.), concentration: 70 g/L, temperature 85° C.,     immersion: 3 minutes -   (h) Drying: 100° C., 10 minutes -   (i) Amount of dry coating film: 10 g/m²

(2-4) Evaluation Test (2-4-1) Workability (Spike Property) Test

A spike test was performed as a test for simulating forward extrusion. The spike test was performed in accordance with the method defined in JP 05-7969 A. After the test, lubricity was evaluated by a spike height and a forming load. The more the spike height increases and the more the forming load decreases, the more lubricity becomes excellent. As mentioned in the above document, an area expansion ratio in the spike test is about 10 times.

-   Lubricity of the film was evaluated by measuring the load and the     spike height during working. -   Test piece for evaluation: S45C spheroidizing-annealed material in     size of 25 mmφ×30 mm

Evaluation Criteria

-   A: Extremely excellent as compared with a phosphate/soap coating     film -   B: Excellent as compared with a phosphate/soap coating film -   C: Equivalent to a phosphate/soap coating film -   D: Inferior as compared with a phosphate/soap coating film -   E: Drastically inferior as compared with a phosphate/soap coating     film

(2-4-2) Workability (Upsetting-Ball Ironing Property) Test

An upsetting-ball ironing test was performed as a test for simulating forming of a bolt head. The upsetting-ball ironing test was performed in accordance with the method defined in JP 2013-215773 A. An area expansion ratio in the upsetting-ball ironing test was adjusted to at most 150 times, and the area expansion ratio is very large as compared with the above-mentioned spike test. Therefore, it is a test capable of reproducing working that requires high workability for formation of a head part of a hexagon bolt with flange. Seizure resistance of the coating film was evaluated by evaluating the amount of seizure of an ironing surface.

-   Test piece for evaluation: S10C spheroidizing-annealed material in     size of 14 mmφ×32 mm -   Bearing ball: 10 mmφ× SUJ2

Evaluation Criteria

Evaluation was made of the area where seizure occurred based on the area of the entire ironing surface.

-   A: Extremely excellent as compared with a phosphate/soap coating     film -   B: Excellent as compared with a phosphate/soap coating film -   C: Equivalent to a phosphate/soap coating film -   D: Inferior as compared with a phosphate/soap coating film -   E: Drastically inferior as compared with a phosphate/soap coating     film

(2-4-3) Evaluation of Corrosion Resistance (Long-Term Rust Prevention Property)

In summer season, a test piece subjected to the above-mentioned coating film treatment was exposed to an open air atmosphere indoors for two weeks or four months, and then the degree of rusting was observed. It was judged that the more the rust area increases, the more corrosion resistance becomes inferior.

-   Test piece: SPCC-SD in size of 75 mm×35 mm×0.8 mm     Evaluation criteria: -   A: Extremely excellent as compared with a phosphate/soap coating     film (rust area of less than 5%) -   B: Excellent as compared with a phosphate/soap coating film (rust     area of 5% or more and less than 15%) -   C: Identical to a phosphate/soap coating film (rust area of 15% or     more and less than 25%) -   D: Inferior to a phosphate/soap coating film (rust area of 25% or     more and less than 35%) -   E: Drastically inferior as compared with a phosphate/soap coating     film (rust area of 35% or more)

The test results are shown in Table 4. As is apparent from Table 4, Examples exhibited satisfactory workability (spike test, ball ironing test), and corrosion resistance (particularly long-term rust prevention property). In Comparative Example 19 in which a coating film is composed only of a zirconium coating film, workability and corrosion resistance were drastically inferior. In Comparative Examples 20 and 21, a film thickness of a zirconium coating film was set at the value deviating from the scope of the present invention. In Comparative Example 20 in which a film thickness of a lower layer coating film was made to be too small, corrosion resistance was inferior. In Comparative Example 21 in which a film thickness of a lower layer coating film was made to be too large, workability was inferior. In Comparative Examples 22 to 29, a mass ratio of silicon to tungsten was set at the value deviating from the scope of the present invention, ball ironing property and corrosion resistance tended to be inferior. In Comparative Examples 30 to 33 in which components other than a water-soluble silicate and a water-soluble tungstate were included as an aqueous inorganic salt, ball ironing property and corrosion resistance were inferior. In Comparative Example 34 in which a coating film of a silicate was formed as a lower layer coating film, it was impossible to obtain high corrosion resistance comparable to Examples. In Comparative Example 35, a coating film of a phosphate was formed as a lower layer coating film. Workability and corrosion resistance are in the same level as in Examples, but this film is not preferred since it contains phosphorus to cause the above-mentioned problem such as phosphorizing of a bolt. In Comparative Example 36 in which a phosphate coating film was subjected to a reactive soap treatment, corrosion resistance was inferior as compared with Examples. Like Comparative Example 35, a phosphate film is not preferred because of a problem such as phosphorizing of a bolt. Regarding phosphorizing, like Comparative Examples 27 and 30, the same applies to the case of containing a phosphate as a water-soluble salt.

TABLE 4 Workability Workability Corrosion Corrosion (Spike (Ball ironing resistance resistance Phosphatizing property) property) (two weeks) (four months) property*1 Example 13 A A A A B Example 14 A B A A B Example 15 A B A A B Example 16 B A A A B Example 17 A A A A B Example 18 A B A A B Example 19 A A A A B Example 20 A A A A B Example 21 A A A B B Example 22 B B A A B Example 23 B A A A B Example 24 B A A A B Example 25 B B A A B Example 26 B C A A B Example 27 B C A A B Example 28 A A A A B Example 29 A B A A B Comparative Example 19 E E E E B Comparative Example 20 A A C C B Comparative Example 21 D C A A B Comparative Example 22 A D D D B Comparative Example 23 A D D D B Comparative Example 24 A D B D B Comparative Example 25 A D B D B Comparative Example 26 B D E E B Comparative Example 27 B D D D E Comparative Example 28 A D D D B Comparative Example 29 B D D D B Comparative Example 30 B D D D E Comparative Example 31 C D E E B Comparative Example 32 A D D D B Comparative Example 33 B E D D B Comparative Example 34 B B C C B Comparative Example 35 B B B B E Comparative Example 36 B B C C E *1B: There is no possibility of brittle fracture of steel wire rod due to phosphorus because of containing no phosphorus. E: There is possibility of brittle fracture of a steel wire rod due to phosphorus because of containing phosphorus.

As is apparent from the above description, when using a steel wire rod having a lubricating coating film, which includes an upper layer coating film and a lower layer coating film, of the present invention, it is possible to achieve both high workability and sufficient corrosion resistance in service environment. Therefore, the present invention has extremely high industrial value of utilization.

The present invention includes the following aspects.

Aspect 1:

A steel wire rod having a coating film containing no phosphorus, wherein

the coating film includes a lower layer coating film composed of hydroxide of zirconium and having a film thickness of 1.0 to 200 nm, and an upper layer coating film containing silicon and tungsten, in order from a steel wire rod side, a mass ratio of tungsten/silicon being in a range of 1.3 to 18.

Aspect 2:

The steel wire rod according to the aspect 1, wherein the silicon is derived from water-soluble silicate, and the tungsten is derived from water-soluble tungstate.

Aspect 3:

The steel wire rod according to the aspect 1 or 2, wherein the silicon is derived from at least one selected from lithium silicate, sodium silicate and potassium silicate, and the tungsten is derived from at least one selected from lithium tungstate, sodium tungstate, potassium tungstate and ammonium tungstate.

Aspect 4:

The steel wire rod according to any one of the aspects 1 to 3, wherein a resin is contained in the upper layer coating film, and a mass ratio of resin/(silicon+tungsten) is in a range of 0.01 to 3.2.

Aspect 5:

The steel wire rod according to the aspect 4, wherein the resin is at least one selected from a vinyl resin, an acrylic resin, an epoxy resin, a urethane resin, a phenol resin, a cellulose derivative, a polymaleic acid and a polyester resin.

Aspect 6:

The steel wire rod according to any one of the aspects 1 to 5, wherein a lubricant is contained in the upper layer coating film, and a mass ratio of lubricant/(silicon+tungsten) is in a range of 0.01 to 3.2.

7.

The steel wire rod according to the aspect 6, wherein the lubricant is at least one selected from wax, polytetrafluoroethylene, fatty acid soap, fatty acid metal soap, fatty acid amide, molybdenum disulfide, tungsten disulfide, graphite and melamine cyanurate.

Aspect 8:

The steel wire rod according to any one of the aspects 1 to 7, wherein the mass of the coating film per unit area of the upper layer coating film is in a range from 1.0 to 20 g/m².

Aspect 9:

A method for producing the steel wire rod according to any one of the aspects 1 to 8, which comprises bringing an aqueous chemical conversion treatment solution, which has a pH in a range of 2.5 to 5.0 and contains a water-soluble zirconium compound dissolved therein, into contact with a surface of a steel wire rod to form a lower layer coating film.

This application claims priority based on Japanese Patent Application No. 2014-070445, filed on Mar. 28, 2014, the disclosure of which is incorporated by reference herein 

1. A steel wire rod comprising a coating film comprising no phosphorus, wherein the coating film comprises a lower layer coating film comprising an oxide and/or hydroxide of zirconium and having a film thickness of 1.0 to 200 nm, and an upper layer coating film comprising silicon and tungsten, in order from a steel wire rod side, wherein a mass ratio of tungsten/silicon is in a range of 1.3 to
 18. 2. The steel wire rod according to claim 1, wherein the silicon is derived from a water-soluble silicate, and the tungsten is derived from a water-soluble tungstate.
 3. The steel wire rod according to claim 1, wherein the silicon is derived from at least one selected from the group consisting of lithium silicate, sodium silicate and potassium silicate, and the tungsten is derived from at least one selected from the group consisting of lithium tungstate, sodium tungstate, potassium tungstate and ammonium tungstate.
 4. The steel wire rod according to claim 1, wherein the upper layer coating film further comprises a resin, and a mass ratio of resin/(silicon+tungsten) is in a range of 0.01 to 3.2.
 5. The steel wire rod according to claim 4, wherein the resin is at least one selected from the group consisting of a vinyl resin, an acrylic resin, an epoxy resin, a urethane resin, a phenol resin, a cellulose derivative, a polymaleic acid and a polyester resin.
 6. The steel wire rod according to claim 1, wherein the upper layer coating film further comprises a lubricant, and a mass ratio of lubricant/(silicon+tungsten) is in a range of 0.01 to 3.2.
 7. The steel wire rod according to claim 6, wherein the lubricant is at least one selected from the group consisting of a wax, polytetrafluoroethylene, a fatty acid soap, a fatty acid metal soap, a fatty acid amide, molybdenum disulfide, tungsten disulfide, graphite and melamine cyanurate.
 8. The steel wire rod according to claim 1, wherein a mass of the coating film per unit area of the upper layer coating film is in a range from 1.0 to 20 g/m².
 9. A method for producing the steel wire rod according to claim 1, which comprises bringing an aqueous chemical conversion treatment solution, which has a pH in a range of 2.5 to 5.0 and contains comprises a water-soluble zirconium compound dissolved therein, into contact with a surface of a steel wire rod to form a lower layer coating film. 